1. Field of the Invention
This invention relates generally to investigation of subsurface earth formations, and, more particularly, to techniques for determining formation properties using a well tool equipped with radiation sources and adapted for conveyance into and retrieval from a downhole tubular.
2. Description of Related Art
Resistivity and gamma-ray logging are the two formation evaluation measurements run most often in well logging. Such measurements are used to locate and evaluate the properties of potential hydrocarbon bearing zones in sub-surface formations. In many wells, they are the only two measurements performed, particularly in low cost wells and in surface and intermediate sections of more expensive wells.
These logging techniques are realized in different ways. A well tool, comprising a number of transmitting and detecting devices for measuring various parameters, can be lowered into a borehole on the end of a cable, or wireline. The cable, which is attached to some sort of mobile processing center at the surface, is the means by which parameter data is sent up to the surface. With this type of wireline logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is being pulled uphole.
Some wells may not be logged because wireline logging is too expensive, when rig time is included in the total cost. Conditioning the well for wireline logging, rigging up the wireline tools, and the time to run the wireline tools in and out require rig time. Horizontal or deviated wells also present increased cost and difficulty for the use of wireline tools.
An alternative to wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance. Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as Measurement While Drilling (MWD) techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are know as Logging While Drilling (LWD). As with wireline logging, the use of LWD and MWD tools may not be justified due to the cost of the equipment and the associated service since the tools are in the hole for the entire time it takes to drill the section.
Logging While Tripping (LWT) presents a cost-effective alternative to LWD and MWD techniques. In LWT, a small diameter “run-in” tool is sent downhole through the drill pipe, at the end of a bit run, just before the drill pipe is pulled. The run-in tool is used to measure the downhole physical quantities as the drill string is extracted or tripped out of the hole. Measured data is recorded into tool memory versus time during the trip out. At the surface, face, a second set of equipment records bit depth versus time for the trip out, and this allows the measurements to be placed on depth.
U.S. Pat. No. 5,589,825 describes a LWT technique incorporating a logging tool adapted for movement through a drillstring and into a drilling sub. The '825 patent describes a sub incorporating a window mechanism to permit signal communication between a housed logging tool and the wellbore. The window mechanism is operable between an open and closed position. A disadvantage of the proposed apparatus is that the open-window mechanism directly exposes the logging tool to the rugose and abrasive borehole environment, where formation cuttings are likely to damage the logging tool and jam the window mechanism. Downhole conditions progressively become more hostile at greater depths. At depths of 5,000 to 8,000 meters, bottom hole temperatures of 260° C. and pressures of 170 Mpa are often encountered. This exacerbates degradation of external or exposed logging tool components. Thus, an open-window structure is impractical for use in a downhole environment.
UK Patent Application GB 2337546A describes a composite structure incorporated within a drill collar to permit the passage of electromagnetic energy for use in measurements during the drilling operation. The '546 application describes a drill collar having voids or recesses with embedded composite covers. A disadvantage of the apparatus proposed by the '546 application is the use of composite materials as an integral part of the drill collar. Fatigue loading (i.e., the bending and rotating of the drill pipe) becomes an issue in drilling operations. When the drill pipe is subjected to bending or torsion, the shapes of the voids or recesses change, resulting in stress failure and poor sealing. The differences in material properties between the metal and composite covers are difficult to manage properly where the composite and metal are required to act mechanically as one piece, such as described in the '546 application. Thus, the increased propensity for failure under the extreme stresses and loading encountered during drilling operations makes implementation of the described structure impractical.
U.S. Pat. Nos. 5,988,300 and 5,944,124 describe a composite tube structure adapted for use in a drillstring. The '300 and '124 patents describe a piecewise structure including a composite tube assembled with end-fittings and an outer wrapping connecting the tube with the end-fittings. In addition to high manufacturing costs, another disadvantage of this structure is that the multi-part assembly is more prone to failure under the extreme stresses encountered during drilling operations.
U.S. Pat. No. 5,939,885 describes a well logging apparatus including a mounting member equipped with coil antennas and housed within a slotted drill collar. However, the apparatus is not designed for LWT operations. U.S. Pat. Nos. 4,041,780 and 4,047,430 describe a logging instrument that is pumped down into a drill pipe for obtaining logging samples. However, the system proposed by the '780 and '430 patents requires the withdrawal of the entire drill string (for removal of the drill bit) before any logging may be commenced. Thus, implementation of the described system is impractical and not cost effective for many operations.
U.S. Pat. No. 5,560,437 describes a telemetry method and apparatus for obtaining measurements of downhole parameters. The '437 patent describes a logging probe that is ejected into the drill string. The logging probe includes a sensor at one end that is positioned through an aperture in a special drill bit at the end of the drill string. As such, the sensor has direct access to the drill bore. A disadvantage of the apparatus proposed by the '437 patent is the sensor's direct exposure to the damaging conditions encountered downhole. The use of a small probe protruding through a small aperture is also impractical for resistivity logging.
U.S. Pat. No. 4,914,637 describes a downhole tool adapted for deployment from the surface through the drill string to a desired location in the conduit. A modulator on the tool transmits gathered signal data to the surface. U.S. Pat. No. 5,050,675 (assigned to the present assignee) describes a perforating apparatus incorporating an inductive coupler configuration for signal communication between the surface and the downhole tool. U.S. Pat. No. 5,455,573 describes an inductive coupling device for coaxially arranged downhole tools. Downhole techniques have also been proposed utilizing slotted tubes. U.S. Pat. No. 5,372,208 describes the use of slotted tube sections as part of a drill string to sample ground water during drilling. However, none of these proposed systems relate to through-tubing measurement or signal transfer.
While LWT techniques incorporating neutron and gamma-ray measurements have been proposed in the market, these designs are not optimized for formation evaluation. A need remains for simplified, reliable, and retrievable radiation-source systems for locating and evaluating the properties of potential hydrocarbon bearing zones in subsurface formations.